The method of the aforementioned type is intended to treat cracked gas in order to be able to extract more than 99.5% by moles of the ethylene contained in the cracked gas, and to obtain an ethylene-rich cut has a content greater than 99.95% by moles of ethylene.
A method of the aforementioned type with which such performances may be obtained is for example described in U.S. Pat. No. 5,253,479.
This method is applied for treating very large volumes of cracked gas, for example greater than 50 metric tons per hour, notably greater than 100 metric tons per hour.
In order to guarantee both very high purity of the produced ethylene stream and a maximum ethylene recovery rate, it is necessary to cool the cracked gas stepwise down to temperatures below −100° C. and notably below −120° C.
For this purpose, the cracked gas is successively cooled in increasingly cold heat exchange regions. The cracked gas is partly condensed in each heat exchange region.
At the outlet of each heat exchange region, the condensed liquid containing the C2+ hydrocarbons is recovered.
The liquids condensed at a higher temperature are sent into an upstream demethanization column in order to recover at the bottom a first cut rich in C2+ hydrocarbons.
The intermediate and downstream liquids obtained at lower temperatures are sent into an intermediate demethanization column which produces at the bottom a second cut rich in C2+ hydrocarbons.
The head stream from the upstream column is introduced into the intermediate column.
In U.S. Pat. No. 5,253,479, in order to further improve recovery of ethylene, the head stream from the intermediate demethanization column is introduced, after cooling, into a third separation column. The bottom of the third column is then partly reintroduced after pumping in a first cryogenic pump, with reflux into the intermediate column. The head of the third column is introduced after cooling and pumping by a second cryogenic pump, into an ethylene absorber which forms the fourth distillation column.
The method described in U.S. Pat. No. 5,253,479 is therefore particularly effective in order to obtain excellent recovery of ethylene.
Taking into account the presence of four distillation columns, and of two cryogenic pumps, the structure of the installation and the energy consumption of the method may nevertheless still be improved.